center columns in readme table

README.md

@@ -3,7 +3,7 @@ Repository for the code to replicate the paper of Shi et al. with our SPECT/CT S
 
 ## Differences 
 |                       |                        Authors |                              Ours |
-| :-------------------- | ------------------------------ | --------------------------------- |
+| :-------------------- | :----------------------------: | :-------------------------------: |
 | SPECT/CT Scanner      |          GE NM/CT 850 SPECT/CT | Siemens Symbia Intevo 16 SPECT/CT |
 | Scan Arc              |                           180° |                              208° |
 | Angles                |                             60 |                                34 |