Typo in INSTALL.armv7

[Eric Lalonde]

--- INSTALL.armv7.orig  2015-09-08 09:48:21.000000000 -0700
+++ INSTALL.armv7       2015-09-08 09:48:35.000000000 -0700
@@ -91,7 +91,7 @@ out as follows:
                        A miniroot filesystem to be used for
                        installation; Cubieboard1 version.

-       miniroot-cubox--58.fs
+       miniroot-cubox-58.fs
                        A miniroot filesystem to be used for
                        installation; CuBox-i version.