docs: correct the descriptions of how to invoke the spell checker

The keystrokes to invoke it changed in version 5.0.

doc/nano.1

@@ -67,7 +67,7 @@ When a more precise piece of text needs to be cut or copied, you can mark
 its start with \fB^6\fR, move the cursor to its end (the marked text will be
 highlighted), and then use \fB^K\fR to cut it, or \fBM\-6\fR to copy it to the
 cutbuffer.  You can also save the marked text to a file with \fB^O\fR, or
-spell check it with \fB^T\fR.
+spell check it with \fB^T^T\fR.
 .sp
 On some terminals, text can be selected also by holding down Shift while
 using the arrow keys.  Holding down the Ctrl or Alt key too will increase

doc/nano.texi

@@ -591,7 +591,7 @@ or @kbd{M-A} and then moving the cursor to the other end of the portion
 to be selected.  The selected portion of text will be highlighted.
 This selection can now be cut or copied in its entirety with a single
 @kbd{^K} or @kbd{M-6}.  Or the selection can be used to limit the scope of
-a search-and-replace (@kbd{^\}) or spell-checking session (@kbd{^T}).
+a search-and-replace (@kbd{^\}) or spell-checking session (@kbd{^T^T}).
 
 On some terminals, text can be selected also by holding down @kbd{Shift}
 while using the cursor keys.  Holding down the @kbd{Ctrl} or @kbd{Alt}