docs: filesystems: ext4: blockgroup.rst: replace some characters

The conversion tools used during DocBook/LaTeX/html/Markdown->ReST
conversion and some cut-and-pasted text contain some characters that
aren't easily reachable on standard keyboards and/or could cause
troubles when parsed by the documentation build system.

Replace the occurences of the following characters:

- U+2217 ('∗'): ASTERISK OPERATOR
use ASCII asterisk instead of the ASTERISK OPERATOR

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Acked-by: Theodore Ts'o <[e-mail]>
Link: https://lore.kernel.org/r/c5c3c3[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/ext4/blockgroup.rst

···
 descriptors copies are kept in the first block group. Given the default
 128MiB(2^27 bytes) block group size and 64-byte group descriptors, ext4
 can have at most 2^27/64 = 2^21 block groups. This limits the entire
-filesystem size to 2^21 ∗ 2^27 = 2^48bytes or 256TiB.
+filesystem size to 2^21 * 2^27 = 2^48bytes or 256TiB.
 
 The solution to this problem is to use the metablock group feature
 (META\_BG), which is already in ext3 for all 2.6 releases. With the