add a comma

book/01_tx_lines.md

@@ -126,7 +126,7 @@ I have mostly seen VSWR used for antennas and system interface specifications. I
 
 ## Return Loss
 
-Another way to look at reflections is with return loss. This is really just a decibel way to look at reflected power. Remember that $\Gamma$ is a voltage ratio, not power ratio so return loss is:
+Another way to look at reflections is with return loss. This is really just a decibel way to look at reflected power. Remember that $\Gamma$ is a voltage ratio, not power ratio, so return loss is:
 
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