Update default log file size in doc.

The default size was changed in #f779e7a5 but the documentation was
never updated.

This fixes #566 reported on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194547959

doc/impl.md

@@ -64,7 +64,7 @@ Other files used for miscellaneous purposes may also be present (LOCK, *.dbtmp).
 
 ## Level 0
 
-When the log file grows above a certain size (1MB by default):
+When the log file grows above a certain size (4MB by default):
 Create a brand new memtable and log file and direct future updates here
 In the background:
 Write the contents of the previous memtable to an sstable