Fix capitalisation of Makefile in CODEOWNERS

Additional reviewers were being requested, because the lower case rule
was not being matched against the actual file name.

Author: thomasspriggs

CODEOWNERS

@@ -56,7 +56,7 @@
 /src/cpp/ @kroening @tautschnig @peterschrammel
 /src/solvers/smt2 @kroening @martin-cs @peterschrammel @thomasspriggs @NlightNFotis @TGWDB
 /src/solvers/smt2_incremental @peterschrammel @thomasspriggs @NlightNFotis @TGWDB @chrisr-diffblue
-/src/solvers/makefile @kroening @tautschnig @peterschrammel @chrisr-diffblue @thomasspriggs @NlightNFotis @TGWDB
+/src/solvers/Makefile @kroening @tautschnig @peterschrammel @chrisr-diffblue @thomasspriggs @NlightNFotis @TGWDB
 /src/statement-list/ @kroening @tautschnig @peterschrammel @pkesseli
 
 /cmake/        @diffblue/diffblue-opensource