docs: fix "availabilty" typo in the manual and the nanorc manpage

doc/nano.texi

@@ -1365,7 +1365,7 @@ Unbinds @code{key} from @code{menu}
 
 Note that @code{bind key "@{function@}" menu} is equivalent to
 @code{bind key function menu}, except that for the latter form
-@command{nano} will check the availabilty of the @code{function}
+@command{nano} will check the availability of the @code{function}
 in the given @code{menu} at startup time (and report an error if
 it does not exist there), whereas for the first form @command{nano}
 will check at execution time that the @code{function} exists but not

doc/nanorc.5

@@ -570,7 +570,7 @@ menus where the key exists when \fBall\fP is used).
 .sp
 Note that \fBbind \fIkey\fR \fB"{\fIfunction\fB}"\fR \fImenu\fR is equivalent
 to \fBbind \fIkey\fR \fIfunction\fR \fImenu\fR, except that for the latter form
-\fBnano\fR will check the availabilty of the \fIfunction\fR in the given \fImenu\fR
+\fBnano\fR will check the availability of the \fIfunction\fR in the given \fImenu\fR
 at startup time (and report an error if it does not exist there), whereas for the
 first form \fBnano\fR will check at execution time that the \fIfunction\fR exists
 but not whether it makes any sense in the current menu.  The user has to take care