Small typo fix in INSTALL.md for NTL configure command. homenc#375

Small typo fix in INSTALL.md for NTL configure command. 
NTL uses the lowercase typeset for `on` or `off` in its configuration scripts.

Author: AlexanderViand

INSTALL.md

@@ -186,7 +186,7 @@ You can install NTL as follows:
 3. NTL is configured, built and installed in the standard Unix way (but
 remember to specify the following flags to `configure`):
 ```
-      ./configure NTL_GMP_LIP=ON SHARED=on  NTL_THREADS=on NTL_THREAD_BOOST=on
+      ./configure NTL_GMP_LIP=on SHARED=on  NTL_THREADS=on NTL_THREAD_BOOST=on
       make
       sudo make install
 ```