Fix documentation inconsistencies

[xxxxme]

In README.md, there are the following typos/inconsistencies:

1. "Some numbers might be two large" should be "Some numbers might be too large" (lines 68 - 69)
2. Directly under the Encoding Data heading, it states the module supports "all four encodings for data" (line 64) and "The only unimplemented encoding is ECI mode" (line 92), but later it is stated, "three of the five possible encodings have been written" (line 119) .
3. The options for the error parameter are listed as "L, M, Q, or H; each level can correct up to 7, 15, 25, or 30 percent of the data respectively" (lines 103 - 104) but later it says, "The code below constructs a QR code with 25% error correction [...] big_code = pyqrcode.create('0987654321', error='L' " (lines 124 & 128). Either line 124 should be changed to "7%" or line 128 should be changed to "error='Q' ".