[mlir][doc] Correct method names in DialectConversion.md to match the code.

[hanchung]

not sure if this is correct, but the method in DialectConversion.h doesn't have s...

[hanchung]

Just found that other methods have the same stuff, feel free to close the patch if this is intended. Or I can update other methods in the doc as well.

[rriddle]

In D92416#2426339, @hanchung wrote:

Just found that other methods have the same stuff, feel free to close the patch if this is intended. Or I can update other methods in the doc as well.

The docs should reflect the code, so if the docs are wrong they should be updated. Can you also update the other methods as well? Thanks for fixing this!

Side note: Can you add a bit more context(e.g. that this is related to DialectConversion) to your commit title and description?

[hanchung]

rename title and fix more

mlir/docs/DialectConversion.md

 
 ```c++
 struct MyTarget : public ConversionTarget {
   MyTarget(MLIRContext &ctx) : ConversionTarget(ctx) {
     //--------------------------------------------------------------------------
     // Marking an operation as Legal:
 
     /// Mark all operations within the LLVM dialect are legal.
-    addLegalDialects<LLVMDialect>();
+    addLegalDialect<LLVMDialect>();
 
     /// Mark `std.constant` op is always legal on this target.
-    addLegalOps<ConstantOp>();
+    addLegalOp<ConstantOp>();
 
     //--------------------------------------------------------------------------
     // Marking an operation as dynamically legal.
 
     /// Mark all operations within Affine dialect have dynamic legality
     /// constraints.
-    addDynamicallyLegalDialects<AffineDialect>();
+    addDynamicallyLegalDialect<AffineDialect>();
 
     /// Mark `std.return` as dynamically legal.
     addDynamicallyLegalOp<ReturnOp>();
 
     /// Mark `std.return` as dynamically legal, but provide a specific legality
     /// callback.
     addDynamicallyLegalOp<ReturnOp>([](ReturnOp op) { ... });