[phobos] small documentation fixes

[Johannes Pfau]

 Hi,
I found some small bugs (incorrectly nested/closed macros) in the
std.math documentation. I did not want to create a bug report, because
it should be really trivial to fix for everyone with commit access. The
fixes:

line 912: (exp)
- *    $(TD +$(INFIN))          $(TD +$(INFIN)) )
- *    $(TD -$(INFIN))          $(TD +0.0)      )
+ *    $(TR $(TD +$(INFIN))     $(TD +$(INFIN)) )
+ *    $(TR $(TD -$(INFIN))     $(TD +0.0)      )

line 945: (expm1)
- *    $(TD +$(INFIN))          $(TD +$(INFIN))    )
- *    $(TD -$(INFIN))          $(TD -1.0)         )
+ *    $(TR $(TD +$(INFIN))     $(TD +$(INFIN))    )
+ *    $(TR $(TD -$(INFIN))     $(TD -1.0)         )

line 1032: (exp2)
- *    $(TR $(TH x)             $(TH exp2(x)    )
- *    $(TD +$(INFIN))          $(TD +$(INFIN)) )
- *    $(TD -$(INFIN))          $(TD +0.0)      )
+ *    $(TR $(TH x)             $(TH exp2(x))   )
+ *    $(TR $(TD +$(INFIN))     $(TD +$(INFIN)) )
+ *    $(TR $(TD -$(INFIN))     $(TD +0.0)      )

-- 
Johannes Pfau