doc: Fix pandoc error in uftrace.md

Currently make install on some Ubuntu versions failed like below:

    GEN      uftrace.1
  pandoc: blocks is null
  CallStack (from HasCallStack):
    error, called at src/Text/Pandoc/Writers/Man.hs:287:47 in pandoc-1.19.2.4-HbfKWUyODESBIy0vGktOwX:Text.Pandoc.Writers.Man
  Makefile:18: recipe for target 'uftrace.1' failed

It's because the description of --task option has incompatible format in
the pandoc syntax.  We don't need that part so let's get rid of it.

Fixed: #1766

Signed-off-by: Yoojung Nam <haileynam0408@gmail.com>

doc/uftrace.md

@@ -356,7 +356,7 @@ record and `replay` internally. Thus, it describes most regular option in detail
 :   Hide small functions run less than the *TIME*
 
 \--task
-:   [info]: Print task relationship in a tree form instead of the tracing info.
+:   Print task relationship in a tree form instead of the tracing info.
 
 \--task-newline
 :   Interleave a newline when task is changed