r.watershed manual: clarify low mem max amount of pixels

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@64926 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.watershed/front/r.watershed.html

@@ -254,7 +254,8 @@ of <em><a href="r.terraflow.html">r.terraflow</a></em> are several times higher
 
 The upper limit of the <em>ram</em> version is 2 billion
 (2<sup>31</sup> - 1) cells, whereas the upper limit for
-the <em>seg</em> version is 9 billion billion (2<sup>63</sup> - 1)
+the <em>seg</em> version is 9 billion-billion (2<sup>63</sup> - 1 = 
+<a href="https://en.wikipedia.org/wiki/9223372036854775807">9.223372e+18</a>)
 cells.<br>  In some situations, the region size (number of cells) may
 be too large for the amount of time or memory
 available. Running <em>r.watershed</em> may then require use of a