[NFC][mlir] Inclusive language: Replace an instance of master in docs

[quinnp]

Summary

[NFC] As part of using inclusive language within the llvm project, this patch
replaces master with main in SPIR-V.md.

mlir/docs/Dialects/SPIR-V.md

 ### Op definitions
 
 We use [Op Definition Spec][ODS] to define all SPIR-V ops. They are written in
 TableGen syntax and placed in various `*Ops.td` files in the header directory.
 Those `*Ops.td` files are organized according to the instruction categories used
 in the SPIR-V specification, for example, an op belonging to the "Atomics
 Instructions" section is put in the `SPIRVAtomicOps.td` file.
 
-`SPIRVOps.td` serves as the master op definition file that includes all files
+`SPIRVOps.td` serves as the main op definition file that includes all files
 for specific categories.
 
 `SPIRVBase.td` defines common classes and utilities used by various op
 definitions. It contains the TableGen SPIR-V dialect definition, SPIR-V
 versions, known extensions, various SPIR-V enums, TableGen SPIR-V types, and
 base op classes, etc.
 
 Many of the contents in `SPIRVBase.td`, e.g., the opcodes and various enums, and