credential: fix grammar

There was a lot going on behind the scenes when the vulnerability and
possible solutions were discussed. Grammar was not a primary focus,
that's why this slipped in.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

credential.h

@@ -32,7 +32,7 @@ void credential_write(const struct credential *, FILE *);
 /*
  * Parse a url into a credential struct, replacing any existing contents.
  *
- * Ifthe url can't be parsed (e.g., a missing "proto://" component), the
+ * If the url can't be parsed (e.g., a missing "proto://" component), the
  * resulting credential will be empty but we'll still return success from the
  * "gently" form.
  *