fix GCM capitalization

nettle.texinfo

@@ -1472,7 +1472,7 @@ processing them independently with the block cipher (Electronic Code
 Book mode, @acronym{ECB}) leaks information. Besides @acronym{ECB},
 Nettle provides three other modes of operation: Cipher Block Chaining
 (@acronym{CBC}), Counter mode (@acronym{CTR}), and Galois/Counter mode
-(@acronym{gcm}). @acronym{CBC} is widely used, but there are a few
+(@acronym{GCM}). @acronym{CBC} is widely used, but there are a few
 subtle issues of information leakage, see, e.g.,
 @uref{http://www.kb.cert.org/vuls/id/958563, @acronym{SSH} @acronym{CBC}
 vulnerability}. @acronym{CTR} and @acronym{GCM}