Fix note about B-prefixed issue labels
Fix note about B-prefixed issue labels

'B' labels are for blockers, not issues which belong elsewhere.

Prompted by this comment on Reddit:
https://www.reddit.com/r/rust/comments/40nzkd/contributing_to_the_rust_compiler_a_short_guide/cyxcjtd

GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust