[Trivial] start the help texts with lowercase

Zcash: Also include AC_HELP_STRING -> AS_HELP_STRING (ostensibly from merging
bitcoin/bitcoin#6317 but the change only occurs in the merge commit
ca5e2a18648cdc0f2a756e7d549f509adce25b00, not the PR itself).

configure.ac

@@ -151,8 +151,8 @@ AC_ARG_ENABLE([glibc-back-compat],
   [use_glibc_compat=no])
 
 AC_ARG_ENABLE([zmq],
-  [AC_HELP_STRING([--disable-zmq],
-  [Disable ZMQ notifications])],
+  [AS_HELP_STRING([--disable-zmq],
+  [disable ZMQ notifications])],
   [use_zmq=$enableval],
   [use_zmq=yes])