pgsql: Reindent some comments

Reindent some comments

Most (older) comments in md.c and smgr.c are indented with a leading
tab on all lines, which isn't the current style and makes updating the
comments a bit annoying. This reindents all these lines with a single
space, as is the normal style. This issue exists in various shapes
throughout the code but it's pretty consistent here, and since there
is a patch pending to refresh some of the comments in these files, it
seems sensible to clean this up here separately.

Discussion: https://www.postgresql.org/message-id/flat/22fed8ba-01c3-2008-a256-4ea912d68fab%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0b8ace8d773257fffeaceda196ed94877c2b74df

Modified Files
--------------
src/backend/storage/smgr/md.c | 128 +++++++++++++++---------------
src/backend/storage/smgr/smgr.c | 170 ++++++++++++++++++++--------------------
2 files changed, 149 insertions(+), 149 deletions(-)