```fix typos (#145)

```
```diff --git a/double-conversion/bignum-dtoa.cc b/double-conversion/bignum-dtoa.cc
index abdd714..dfd159d 100644
--- a/double-conversion/bignum-dtoa.cc
+++ b/double-conversion/bignum-dtoa.cc
```
```@@ -370,7 +370,7 @@
// Returns an estimation of k such that 10^(k-1) <= v < 10^k where
// v = f * 2^exponent and 2^52 <= f < 2^53.
// v is hence a normalized double with the given exponent. The output is an
-// approximation for the exponent of the decimal approimation .digits * 10^k.
+// approximation for the exponent of the decimal approximation .digits * 10^k.
//
// The result might undershoot by 1 in which case 10^k <= v < 10^k+1.
// Note: this property holds for v's upper boundary m+ too.
@@ -548,7 +548,7 @@
//
// Let ep == estimated_power, then the returned values will satisfy:
//  v / 10^ep = numerator / denominator.
-//  v's boundarys m- and m+:
+//  v's boundaries m- and m+:
//    m- / 10^ep == v / 10^ep - delta_minus / denominator
//    m+ / 10^ep == v / 10^ep + delta_plus / denominator
//  Or in other words:
```