[mlir][doc] Fix links and indentation of mlir::ModuleOp description

[zero9178]

Summary

All lines after the first are currently indented by one char further to the left than the first line. This leads to the first character of each sentence being cut from the resulting Markdown file after compilation. The text also contains 3 references to sections of other markdown files. One was missing the file, while the other two had outdated files, leading to 404 errors in the documentation.

Current text on https://mlir.llvm.org/docs/Dialects/Builtin/ before this patch for reference:

A module represents a top-level container operation. It contains a single graph region](#control-flow-and-ssacfg-regions) containing a single block hich can contain any operations and does not have a terminator. Operations ithin this region cannot implicitly capture values defined outside the module, .e. Modules are IsolatedFromAbove . Modules have n optional symbol name which can be used to refer o them in operations.

mlir/include/mlir/IR/BuiltinOps.td

 //===----------------------------------------------------------------------===//
 
 def ModuleOp : Builtin_Op<"module", [
   AffineScope, IsolatedFromAbove, NoRegionArguments, SymbolTable, Symbol] 
   # GraphRegionNoTerminator.traits> {
   let summary = "A top level container operation";
   let description = [{
     A `module` represents a top-level container operation. It contains a single
-   [graph region](#control-flow-and-ssacfg-regions) containing a single block
+    [graph region](../LangRef.md#control-flow-and-ssacfg-regions) containing a single block
     which can contain any operations and does not have a terminator. Operations
     within this region cannot implicitly capture values defined outside the module,
-   i.e. Modules are [IsolatedFromAbove](Traits.md#isolatedfromabove). Modules have
-   an optional [symbol name](SymbolsAndSymbolTables.md) which can be used to refer
+    i.e. Modules are [IsolatedFromAbove](../Traits.md#isolatedfromabove). Modules have
+    an optional [symbol name](../SymbolsAndSymbolTables.md) which can be used to refer
     to them in operations.
 
     Example:
 
     ```mlir
     module {
       func @foo()
     }
     ```