s/minor/major/ in API cleanups doc.

doc/core/pending_api_cleanups.md

 There are times when we make changes that include a temporary shim for
 backward-compatibility (e.g., a macro or some other function to preserve
-the original API) to avoid having to bump the minor version number in
+the original API) to avoid having to bump the major version number in
 the next release.  However, when we do eventually want to release a
 feature that does change the API in a non-backward-compatible way, we
-will wind up bumping the minor version number anyway, at which point we
+will wind up bumping the major version number anyway, at which point we
 can take the opportunity to clean up any pending backward-compatibility
 shims.
 
 This file lists all pending backward-compatibility changes that should
-be cleaned up the next time we are going to bump the minor version
+be cleaned up the next time we are going to bump the major version
 number:
 
 - remove `GRPC_ARG_MAX_MESSAGE_LENGTH` channel arg from