fix build of crypt.pdf

Author: sjaeckel

doc/crypt.tex

@@ -1365,8 +1365,8 @@ \chapter{Stream Ciphers}
 and if set to 0 the default number of rounds, 20, will be used.
 \vspace{1mm}
 
-If you define \textit{LTC_XSALSA20} to include \textit{XSalsa20} in a minimal
-\textit{libtomcrypt} library build, you must also define \textit{LTC_SALSA20}.
+If you define \textit{LTC\_XSALSA20} to include \textit{XSalsa20} in a minimal
+\textit{libtomcrypt} library build, you must also define \textit{LTC\_SALSA20}.
 \vspace{1mm}
 
 As always, never ever use the same key + nonce/IV pair more than once.