Fix inconsistent usage example in the help message

In the 'Examples' section of the help message, the third usage
example is inconsistent with its description.

    timebox 12      run a 10 minute time box

While the usage example shows how to run a 12 minute time box,
the description mentions that this runs a 10 minute time box.
Correct this usage example to the following.

    timebox 10      run a 10 minute time box

Since this usage message is present in the project README file,
make the same correction in the README file too.

README.md

@@ -84,7 +84,7 @@ Usage
     Examples:
       timebox         run a 30 minute time box
       timebox 15      run a 15 minute time box
-      timebox 12      run a 10 minute time box
+      timebox 10      run a 10 minute time box
 
     Report bugs to <susam@susam.in>.
 

timebox.cmd

@@ -217,7 +217,7 @@ rem OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     echo Examples:
     echo   %NAME%         run a %DD% minute time box
     echo   %NAME% 15      run a 15 minute time box
-    echo   %NAME% 12      run a 10 minute time box
+    echo   %NAME% 10      run a 10 minute time box
     echo.
     echo Report bugs to ^<susam@susam.in^>.
     if %MODE% == Explorer (