Update src/doc/unstable-book/src/language-features/rustc-attrs.md

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: pickfire

src/doc/unstable-book/src/language-features/rustc-attrs.md

@@ -1,6 +1,6 @@
 # `rustc_attrs`
 
-This feature has no tracking issue, and is therefore likely internal to
+This feature has no tracking issue, and is therefore internal to
 the compiler, not being intended for general use.
 
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